The need to measure the torque which is transmitted via a shaft exists both for monitoring and control. The need exists within a plurality of different process industries and for other purposes within devices and products. The present invention is a novel type of magnetoelastic torque transducer for contactless measurement of the transmitted torque in a rotating as well as a stationary shaft.
Generally, a circular cylindrical shaft which is subjected to a torque is influenced by a pure shear stress. This stress state can be expressed, in terms of its principal stresses, as a compressive stress and a tensile stress, directed perpendicularly thereto, of the same magnitude. The principal stress directions are inclined at xc2x145xc2x0 to a generatrix to the cylinder surface.
For most modern contactless magnetoelastic torque transducers, the torque measurement is based on the above principle. The torque in these torque transducers is thus measured by measuring the change of the magnetic permeability in a certain direction in the material constituting a transducer shaft.
The most commonly used torque measurement which makes use of this is designed such that, within a measurement range of the shaft, a rotationally symmetrical, homogeneous magnetizing field, that is, an H-field, is created with the aid of a surrounding stationary excitation winding. This results in an equally homogenous magnetic flux density, that is, a B-field, in the shaft in unloaded state. When the shaft is loaded, the field configuration of the B-field is distorted, which can be detected with the aid of detection windings.
The state of the art as regards the constructive design of torque transducers based on the above method is disclosed in a number of patent specifications. Common to most of these solutions is that two zones are created in the magnetic material, with some type of anisotropy, which causes the magnetic flux density to be deflected at an angle away from its natural direction in parallel with the generatrices to the cylinder surface of the transducer shaft.
SU 667836 describes a method in which the anisotropy is created purely geometrically in each zone by cutting grooves in the surface of the shaft according to a specific pattern. This pattern consists of a number of mutually parallel lines directed at an angle of 45xc2x0 to a generatrix to the cylinder surface of the transducer shaft.
U.S. Pat. No. 4,823,620 describes the same embodiment as above with respect to the geometrical anisotropy, however with the addition that the surface of the shaft is hardened or carburized for the purpose of reducing the hysteresis in the transducer. Both of these methods have their limitations. The methods presuppose that full rotational symmetry of the anisotropic patterns is achieved. The shaft must also be machined to achieve the desired magnetic anisotropy.
Another magnetoelastic method for measuring the torque in a shaft is clear from EP 0 525 551 A2. The shaft whose torque is to be measured is provided with a circularly polarized magnetoelastic ring which is pressed, shrunk or glued onto the shaft. When applying torque to the transducer shaft, the shaft, and hence the ring, will be distorted. This results in the originally purely azimuthal magnetization being reoriented in a helical pattern with both an azimuthal and an axial component. With the aid of a Hall element, freely mounted relative to the shaft, which is oriented in such a way that the Hall element only senses the axial component of the magnetization, a measure of the torque occurring in the shaft is obtained.
Using a Hall element according to the above entails several disadvantages. On the one hand, a Hall element will measure the field at one point only along the circumference of the ring. When the transducer shaft rotates, and hence also the circularly polarized magnetic ring, the output signal from the Hall element will vary unless the magnetic field of the ring has a perfect rotational symmetry. Another disadvantage is that Hall elements have very small signal levels, which makes the signal processing difficult and makes the signal sensitive to electrical interference. In addition, Hall elements have a temperature drift which is not unessential.
In patent application WO 9722866A1, a torque transducer comprising a shaft with a circularly polarized ring is described. The ring is concentrically surrounded by a stationary tubular shell, freely mounted in relation to the shaft, which shell is provided with a winding for magnetization of the shell with a given frequency. A phase-sensitive detector connected to the winding supplies a signal corresponding to the content of the voltages with even harmonics, induced in the winding, for the purpose of measuring the axial component of the magnetic field.
One difficulty with the above solution is that a large part of the axial magnetic flux, which arises in the ring when loading a torque transducer shaft, will inevitably complete the circuit through the transducer shaft in the case of magnetic transducer shaft material. This then implies that the static magnetic flux mentioned, which is intended for generating even harmonics of the time-varying flux in the shell above it, will substantially not reach to the shell.
An accurate measurement of the transmitted torque may be relatively difficult to achieve in the case of rotating shafts since the accuracy may be influenced if the outer shell does not possess a sufficiently large degree of rotational symmetry. In addition, the choice of transducer shaft material is dependent on the magnetic properties of the material and is, in practice, limited to paramagnetic materials only.
One object of the present invention is to achieve a torque transducer for which the choice of transducer shaft material is independent of the magnetic properties of the material. Another object of the invention is to design a torque transducer whose transducer shaft may comprise a ferromagnetic material.
Another object of the invention is to arrange a torque transducer by means of which the axial dc field, which arises in the ring when loading the circularly polarized ring, is utilized more efficiently for generating even harmonics of the flux in the magnetic shell.
Still another object is to arrange a shell and a transducer shaft of a torque transducer in such a way that the angular dependence of the transducer signal becomes minimal. An additional object of the invention is to arrange a torque transducer in which the power losses, due to air leakage in the high-reluctance air path outside the excitation winding of the transducer, are reduced.
Yet another object of the invention is to arrange a torque transducer by means of which a good accuracy of measurement can always be obtained, also when the magnetic field in the magnetoelastic ring has a moderate degree of rotational symmetry. When measuring the transmitted torque in, for example, a rotating shaft, it is of great importance that the accuracy of measurement should be independent of any rotational symmetry of the mentioned magnetic field.
These and other objects are fulfilled according to the invention by a torque transducer for measuring the transmitted torque in a rotating as well as a stationary shaft according to claim 1. Further, some advantageous embodiments of the present invention are described in claims 2-14. The present invention relates to a torque transducer which is arranged such that the transducer shaft is manufactured of an arbitrary material independently of the magnetic properties of the material.
According to the prior art, when applying torque to the transducer shaft, a static magnetic axial flux arises in the transducer ring. The static magnetic axial flux completes the circuit through the air above the ring and through the shaft material below the ring. The proportion of magnetic flux which completes the circuit through the air is approximately proportional to the quotient between the relative magnetic permeability in the air and in the shaft material, respectively.
For non-magnetic materials, with largely the same relative permeability as that of the air, this leads to the return flux substantially completing the circuit in equal parts through the air and the shaft. This implies that only half of the static magnetic flux, the dc flux, is available to interact, in the shell, with the time-varying flux, the ac flux, from the excitation winding for the purpose of inducing even harmonics in the ac flux for measurement. If a torque transducer shaft according to the prior art is made of a ferromagnetic material, the proportion of dc flux in the shell intended for inducing measurable voltages, with even harmonics, will decrease considerably.
The present invention has the advantage that the transducer shaft may be made of an arbitrary material, independently of the magnetic properties of the material, substantially by placing a highly permeable magnetic layer of a magnetic non-linear material with a high relative permeability arranged in physical contact with the cylinder-shaped magnetoelastic region on the transducer shaft. The result of this is that the static magnetic flux, the dc flux, effectively completes the circuit through the magnetic layer since the flux, compared with torque transducers according to the prior art, does not have to be conducted through the air. This, in turn, causes most of the axial static magnetic flux, which upon loading is generated by a magnetic circularly polarized and cylinder-shaped region on the transducer shaft, to be controlled towards the time-varying magnetic flux in the layer, the ac flux, generated by the excitation winding. Because of this, the dc field in the present invention is used in a much more efficient way than according to the prior art.
Torque transducers according to the preferred embodiment, where the transducer shaft may also comprise ferromagnetic materials, are more advantageous for a number of reasons.
One advantage of the present invention is that the transducer shaft may be manufactured from optional construction materials. The term optional materials here means that the shaft on which the torque is to be measured, such as a spindle in a steering gear, a shaft in a gearbox, a drive shaft or a spindle, etc., is manufactured from a material which has the desired material properties to efficiently achieve the objects and performance of the object. A transducer shaft of a ferromagnetic material means that a torque transducer according to the present invention has a considerably wider range of application. In most cases, the use of a ferromagnetic material also entails lower material costs, which in turn involves lower production costs for the transducer as a whole.
Another advantage of a preferred embodiment of the present invention is that the measuring zone of such a transducer shaft need not be machined or processed separately to receive special magnetic properties. Another embodiment of the invention has the advantage that no magnetic circularly polarized ring need be attached to the transducer shaft. According to this embodiment, a substantially cylinder-shaped and magneto-strictive region of the transducer shaft material is provided with a higher magnetic relative permeability for circular polarization, such that the use of a separate ring according to the prior art can be eliminated.
According to the best use of the present invention, an embodiment of the invention is used where the winding is provided with a magnetic return conductor. Such a configuration entails a considerably lower power consumption compared with a corresponding configuration for a torque transducer according to the prior art, which is a significant advantage.